In particular, in the field of the production of IPN polymeric foams consisting of polyamide-polyimide-polyvinylchloride-polyisocyanurate-polyurea (see description of the IPN concept in “Interpenetrating Polymer Networks”, D. Klempner, L. H. Sperling, L. A. Utracki; vol. 239; American Chemical Society; Ed. 1994), the initial mixture of polymers, surfactants, chemical blowing agents, isocyanates, etc., is introduced into the mold. Once the mold has been closed by means of a suitable lid, the mixture is heated, thus causing the generation of blowing gas, the gelation of the polymer and the desired cross-linking reactions, with the incorporation of the gas inside the molecules of cross-linked IPN thus obtained. This part of the process is, in particular, performed in a closed mold and at a constant reaction volume, the pressure inside the mold being generated by the formation of gas and by the thermal expansion.
The known art described herein, however, has the drawback that, due to the expansion induced by the formation of blowing gas and by the heating of the plastic material, the mass of molten polymer and reagents tends to expand, in contrast with the action exerted by the lid which closes the mold. This compressed molten mass therefore tends to expand, leaking through the closed areas of the lid on the mold. Consequently, in traditional production processes which follow this molding technique, considerable quantities of waste material are formed, which are responsible for an undesired increase in the general production costs, in addition to maintenance problems relating to the machine and the environmental impact.
In addition to these disadvantages, there is also the degradation of the microcellular structure of the finished product, particularly in correspondence with the leakage of plastic material from the mold.
A further drawback of the present methods for the production of expanded PVC lies in the high amounts of chemical blowing agents used, which, in addition to being harmful for the environment, are also expensive due to the complexity of the production, transportation and storage of these products.
Publication EP 2039496 A1 discloses a process for the production of rubber, using microspheres suitable for being crushed under the action of the volume increase of the starting mixture inside the vulcanization mold. According to this known technique, which does not relate to the field of expanded plastic materials, the molded product incorporates the microspheres in their final and definitive crushed or collapsed state, which is that of the final product based on rubber.